Kidneys: Function and Structure

The kidneys are essential for regulating the volume and composition of bodily fluids. This page outlines key regulatory systems involving the kidneys for controlling volume, sodium and potassium concentrations, and the pH of bodily fluids.

A most critical concept for you to understand is how water and sodium regulation are integrated to defend the body against all possible disturbances in the volume and osmolarity of bodily fluids. Simple examples of such disturbances include dehydration, blood loss, salt ingestion, and plain water ingestion.

How water balance is regulated by ADH

Water balance is achieved in the body by ensuring that the amount of water consumed in food and drink (and generated by metabolism) equals the amount of water excreted. The consumption side is regulated by behavioural mechanisms, including thirst and salt cravings. While almost a litre of water per day is lost through the skin, lungs, and feces, the kidneys are the major site of regulated excretion of water.

One way the kidneys can directly control the volume of bodily fluids is by the amount of water excreted in the urine. Either the kidneys can conserve water by producing urine that is concentrated relative to plasma, or they can rid the body of excess water by producing urine that is dilute relative to plasma.

Direct control of water excretion in the kidneys is exercised by vasopressin, or anti-diuretic hormone (ADH), a peptide hormone secreted by the hypothalamus. ADH causes the insertion of water channels into the membranes of cells lining the collecting ducts, allowing water reabsorption to occur. Without ADH, little water is reabsorbed in the collecting ducts and dilute urine is excreted.

How the kidney uses a counter current mechanism

Because the human body does not maintain a constant water volume, the kidneys have to compensate for the lack of or excess of water consumed. The kidneys use a transport system called the counter-current mechanism to accomplish this (Hoppensteadt et al, 186). The name is based on the fact that concentration first increases in the direction of flow, then decreases as flow continues through the ascending parallel loop. The mechanism relies on the adjacent, parallel loops of Henle and vasa recta.

In the ascending loop, Na+ or any solute is actively pumped out of the tubule. Because water is impermeable in the ascending loop, the volume at the bottom of the loop is the same as that entering the distal tubule. At the bottom of the loop, the tubular and interstitial concentrations are equal.

In the descending loop, the concentrations inside and outside the tubule are increasing with the current, with the maximum concentration being reached at the bottom of the loop. The increased concentration is the result of the passive diffusion of Na+ into the tubule and water out of the tubule. When the filtrate reaches the distal tubule, a net loss of Na+ and water has occurred through the loops of Henle.

How the PH is controlled by the kidney

The secretion of further substances not required by the body may take place in the distal convoluted tubule, e.g. hydrogen and hydro carbonate ions. This is very important in the control of plasma Ph, which must be maintained at 7.4. If the pH plasma falls, hydrogen ions are excreted by the kidney; if the plasma pH raises hydrogen carbonate ions secreted.

Active Transport

Active transport is the energy-demanding transfer of a substance across a cell membrane against its concentration gradient, i.e., from lower concentration to higher concentration.

Special proteins within the cell membrane act as specific protein ‘carriers’. The energy for active transport comes from ATP generated by respiration (in mitochondria).

Major examples of Active Transport such as:

• Re-absorption of glucose,
• Amino acids
• Salts by the proximal convoluted tubule of the nephron in the kidney.

A mechanism of active transport which move potassium ions into and sodium ions out of a cell along with protein (or enzyme) channel. It is found in all human cells, but is especially important in nerve and muscle cells.

The sodium-potassium pump uses active transport, with energy supplied by ATP (adenosine triphosphate) molecules, to move 3 sodium ions to the outside of the cell for each 2 potassium ions that it moves in. One third of the body’s energy expenditure is used in this process.

Buffer system

The kidneys and the lungs work together to help maintain a blood pH of 7.4 by affecting the components of the buffers in the blood. Therefore, to understand how these organs help control the pH of the blood, we must first discuss how buffers work in solution.

Acid-base buffers confer resistance to a change in the pH of a solution when hydrogen ions (protons) or hydroxide ions are added or removed. An acid-base buffer typically consists of a weak acid, and its base (salt). Buffers work because the concentrations of the weak acid and its salt are large compared to the amount of protons or hydroxide ions added or removed.

When protons are added to the solution from an external source, some of the base component of the buffer is converted to the weak-acid component (therefore, using up most of the protons added); when hydroxide ions are added to the solution (or, equivalently, protons are removed from the solution; protons are dissociated from some of the weak-acid molecules of the buffer, converting them to the base of the buffer (and therefore replenishing most of the protons removed).

However, the change in acid and base concentrations is small relative to the amounts of these species present in solution. Hence, the ratio of acid to base changes only slightly. Thus, the effect on the pH of the solution is small, within certain limitations on the amount of H+ or OH- added or removed.

Other buffers perform a more minor role than the carbonic-acid-bicarbonate buffer in regulating the pH of the blood. The phosphate buffer consists of phosphoric acid (H3PO4) in equilibrium with dihydrogen phosphate ion (H2PO4-) and H+. The pK for the phosphate buffer is 6.8, which allows this buffer to function within its optimal buffering range at physiological pH.

The phosphate buffer only plays a minor role in the blood, however, because H3PO4 and H2PO4- are found in very low concentration in the blood. Haemoglobin also acts as a pH buffer in the blood. Protein can reversibly bind either H+ (to the protein) or O2, but that when one of these substances is bound, the other is released (as explained by the Bohr effect).

During exercise, haemoglobin helps to control the pH of the blood by binding some of the excess protons that are generated in the muscles. At the same time, molecular oxygen is released for use by the muscles.

The symptoms of kidney failure:

There are two types of kidney failure; one of them is acute renal failure and the other type is: Chronic renal failure.

Acute renal failure.

• Blood loss, causing a drop in blood pressure.
• Vomiting and diarrhea, causing dehydration.
• Crush injuries. If large amounts of muscle are damaged there is a release of toxic protein substances that are harmful to the kidneys.
• Sudden blockage of urine drainage.
• Chronic renal failure

The damage to the kidneys is usually ‘silent’ and not noticed at an early stage. It may be discovered incidentally from blood or urine tests done for other reasons. High blood pressure very commonly occurs with it. Symptoms are uncommon unless kidney failure is far advanced, when any of the following may be present:

The symptoms of Chronic renal failure

• Tiredness
• Itching
• Loss of appetite
• Nausea and vomiting
• Breathlessness
• Fluid retention, shown as ankle swelling
• Weakness.

The importance to the body to maintain acid base levels

All the cells that make up the human body are slightly alkaline and the alkalinity must be maintained in order to function and remain healthy. However, their cellular activity creates acid and this acid is what gives the cell energy and function. As each alkaline cell performs its task of respiration, it secrets metabolic wastes and these end products of cellular metabolism are acid in nature.

Although these wastes are used for energy and function, they must not be allowed to build up. An example of this would be the lactic acid which is created through exercise. The body will go to great lengths to neutralise and detoxify these acids before they act as poisons in and around the cell, ultimately changing the environment of the cell.

The human body is very intelligent; as the human body become more acidic the body starts to set up defence mechanisms to keep the damaging acid from entering our organs. It’s known as that the acid gets stored in the fat cells. However, if the acid does come to contact with an organ the acid has a chance to eat holes in the tissue which may cause the cell to mutate (change in a chromosome or a gene).

The oxygen level drops in this acidic environment and calcium begins to be depleted. So as a defense mechanism, our body may actually make fat to protect us from our overly-acidic self. Those fat cells and cellulite deposits may actually be packing up the acid and trying to keep it a safe distance from our organs to safe them from damage.

The effect of exercise on body fluid requirements

Optimal pH of the blood is 7.2, the body will do everything it can to maintain that pH. This is necessary to run the entire body’s biochemical pathways for detoxification, building, and general maintenance. The body has several control mechanisms to keep it at this pH and they include getting rid of excess acid or base by-products through the lungs, saliva and urine.

When the body is sick in any way this pH is disrupted. Most times your body is trying to keep up with the extra acid produced. Acids are produced from lack of oxygen, eating an imbalance of protein and carbohydrates and other acid producing foods, and by cell breakdown and production of metabolic waste.

During exercise, the muscles use up oxygen as they convert chemical energy in glucose to mechanical energy. This O2 comes from hemoglobin in the blood. CO2 and H+ are produced during the breakdown of glucose, and are removed from the muscle via the blood. The production and removal of CO2 and H+, together with the use and transport of O2, cause chemical changes in the blood. These chemical changes, unless offset by other physiological functions, cause the pH of the blood to drop.

If the pH of the body gets too low (below7.4) this result in a condition known as acidosis. This can be very serious, because many of the chemical reactions that occur in the body, especially those involving proteins, are pH-dependent. Ideally, the pH of the blood should be maintained at 7.4. If the pH drops below 6.8 or rises above 7.8, death may occur. Fortunately, we have buffers in the blood to protect against large changes in pH.

Production of CO2 is a result of normal body metabolism. Exercise will increase the production of CO2 through increased respiration in the lungs. When oxygen (O2) is inhaled and CO2 is exhaled, the blood transports these gases to the lungs and body tissues. The body’s metabolism produces acids that are buffered and then excreted by the lungs and kidneys to maintain body fluids at a neutral pH. Disruptions in CO2 levels and HCO3 -create acid-base imbalances. When acid-base imbalances occur, the disturbances can be broadly divided into either acidosis (excess acid) or alkalosis (excess base/alkali).

Urine becomes increasingly acidic as the amount of excess acid retained by the body increases. Alkaline urine, usually containing bicarbonate-carbonic acid buffer, is normally excreted when there is an excess of base or alkali in the body. Secretion of acid or alkaline urine by the kidneys is one of the most important mechanisms the body uses to maintain a constant body pH. As we exercise the urine pH becomes more acidic because the condition which known as acidosis have occurred and this results from a build-up of carbon dioxide in the blood, as well as starvation and dehydration.

As we exercise the temperature increases, and the amount of O2 released from the haemoglobin. Heat is a bi product of the metabolic reactions of all cells and the heat released by contracting muscle fibers tends to raise body temperature. Metabolically active cells require more O2 and liberate more acids and heat.

If we have an increase in temperature, it causes the rate of respiration to increase too. Because O2 tends to be released from the haemoglobin compared to when the weather is cold. This explains why during fever, a person will breathe faster than normal person.

In contrast, during hypothermia (lowered body temperature) cellular metabolism slows and the need for O2 is reduced, and more O2 remains bound to haemoglobin.

Body Adjustment to improve fitness levels

Exercises help our body to adjust and improve its capacity for physical activities. In order to increase our overall fitness level we have to concentrate on three different areas:

• Cardiovascular training
• Strength training
• Flexibility training

Cardiovascular training

Cardiovascular training is aerobic exercise that involves the large muscles like legs and helps make the heart and lungs stronger. Cardiovascular exercise has lots of health benefits like lowering the blood pressure, and also it can burn lots of calories.

This type of exercise leads to improvements in the heart’s ability to pump blood through the body to the working muscles and improves overall cardiovascular health. It is also linked to a number of health improvements including a decreased risk of many diseases, decreases in total cholesterol, blood pressure and levels of body fat.

Strength training

In order to improve our strength, a change is needed to be made, otherwise if we simply lift the same weights, the same way, then we will stay the same – our training is maintenance based. If we want to improve our strength training, then we will need to apply a number of different variations into our workout routines to avoid letting the body become adapted to the current strength training workouts.

A muscle will only strengthen when forced to operate beyond its customary intensity (overload). Overload can be progressed by increasing the:

(1) Resistance e.g. adding more weight. (2) Number of repetitions with a particular weight. (3) Number of sets of the exercise. (4) Intensity, i.e. reducing the recovery periods

Flexibility training

Flexibility is a joint’s ability to move through a full range of motion. Flexibility training, also called flexibility stretching that helps balance muscle groups that might be overused during exercise or physical activity. There are many benefits to flexibility training. Some of the benefits are:

• Improved Physical Performance.
• Decreased Risk of Injury.
• Increased Blood and Nutrients to Tissues.

Stretching increases tissue temperature, which increases circulation and nutrient transport. Increased circulation and nutrient transport allows greater elasticity of surrounding tissues and increases performance.

Maintaining Fluid Balances

Fluid balance defines the state where a body’s required amount of water is present and proportioned normally among the various compartments; this state is inseparable from electrolyte balance. Under normal conditions water loss equals water gain and a body’s water volume remains constant. Avenues for water loss include the kidneys, skin, lungs, feces, and menstruation. Water is sourced mostly from dietary intake; this is called preformed water.

Water is not produced by the body to maintain homeostasis; metabolic water production is simply a by-product of cellular respiration. The body regulates water intake via the thirst reflex which stimulates us to drink. When water loss is greater than water gain the body reaches a state of dehydration, and dehydration stimulates the thirst reflex in three ways:

The level of saliva drops resulting in a dry mucosa in the mouth and pharynx;

There is an increase in blood osmotic pressure which stimulates osmoreceptors in the hypothalamus;

There is a drop in blood volume, which leads to the renin/angiotensin pathway stimulating the thirst centre in the hypothalamus.

When the blood looses excessive fluid dehydration occurs and the blood becomes more viscous (reduce ability to flow). This results in insufficient blood supply to the working muscles. After exercise, a drop in body fluid results in an increase in blood tonicity and a decrease in blood volume which in turn causes the release of renin in the kidneys and stimulation of osmoreceptors in the hypothalamus.Therefore after exercise, the exerciser must focus on the following areas:

Effect of drinks

Cardiovascular and thermoregulatory responses to fluid ingestion

Carbohydrates feeding and exercise performance

Sports drinks must be formulated to taste best when people are hot and sweaty so that they can drink as much as they possibly can. The sports drinks are absorbed faster than plain water during exercise and rest. During exercise fluid consumption is vital for two primary purposes – safe guarding health and optimizing performance

Therefore, we need to consume more carbohydrate which helps maintaining blood glucose and increases carbohydrate oxidation, assure skeletal muscle and CNS sufficient supply of energy.

Sources Used

http://www.enotes.com/nursing-encyclopedia/acid-base-balance

http://www.shodor.org/Master/biomed/physio/dialysis/kidfunc.htm

http://www.8candlesonline.com/purify/what_is/alkalinity.html

http://mcb.berkeley.edu/courses/mcb135e/kidneyfluid.html

http://www.ann.com.au/MedSci/fluid.htm

Books

Essential AS Biology by( Glenn and Susan Toole)

AS Biology by (Pete Kennedy and Frank Sochacki)